This invention relates generally to a method and apparatus for remote imaging of objects enveloped by a backscattering medium which is at least partially transmitting to light. More particularly, this invention relates to a method for detecting and imaging underwater objects from an airborne platform or from a submarine using a LIDAR (light detection and ranging) imaging system which employs a novel filtering scheme to minimize optical noise, particularly solar noise. This invention also relates to laser systems for use in (1) a LIDAR bathymetry system; and (2) submarine communications between both underwater and above-water locations.
It is desirable in a number of military and civilian applications to search a volume within a backscattering medium for the presence of certain targets. For instance, moored or bottom mines deployed in ocean shipping lanes are a hazard to navigating ships used both for military and for commercial purposes. For other civilian applications such as law enforcement on the ocean, it is desirable to detect the presence of submerged fishing nets, drug carrying containers or smuggled contraband. In or near harbors and beaches, it is also desirable to detect submerged obstructions, cables, pipelines, barrels, oil drums, etc. In strictly military applications, anti-submarine warfare requires an effective means of detecting and locating submarines.
Presently, cumbersome and time consuming wire line devices must be used for detecting underwater targets from remote airborne locations. These devices are lowered into the water and of course, are easily subject to damage and loss. Also, wire line devices make target searching relatively slow and can only detect targets without providing visual imaging.
An important and novel system for remote detection and imaging of objects underwater (or objects obscured by other backscattering media which is at least partially transmitting to light such as ice, snow, fog dust and smoke) from an airborne platform has been described in U.S. Pat. No. 4,862,257 and U.S. Pat. No. 5,013,917, both of which are assigned to the assignee hereof and incorporated herein by reference. The imaging LIDAR system of U.S. Pat. No. 4,862,257 utilizes a laser to generate short pulses of light with pulse widths on the order of nanoseconds. The laser light is expanded by optics and projected down toward the surface of the water and to an object or target. U.S. Pat. No. 5,013,917 relates to an imaging LIDAR system intended for night vision.
Imaging LIDAR systems of the type described hereinabove are also disclosed in commonly assigned U.S. Pat. Nos. 4,964,721 and 4,967,270, both of which are assigned to the assignee hereof and fully incorporated herein by reference. U.S. Pat. No. 4,964,721 relates to an imaging LIDAR system which controls camera gating based on input from the aircraft on board altimeter and uses a computer to thereby adjust total time delay so as to automatically track changing platform altitude. U.S. Pat. No. 4,967,270 relates to a LIDAR system employing a plurality of gated cameras which are individually triggered after preselected time delays to obtain multiple subimages laterally across a target image. These multiple subimages are then put together in a mosaic in a computer to provide a complete image of a target plane preferably using only a single light pulse.
U.S. Ser. No. 565,631 filed Aug. 10, 1990 which is also assigned to the assignee hereof and fully incorporated herein by reference, relates to an airborne imaging LIDAR system which employs multiple pulsed laser transmitters, multiple gated and intensified array camera receivers, an optical scanner for increased field of regard, and a computer for system control, automatic target detection and display generation.
U.S. Ser. No. 565,631 provides a means for rapidly searching a large volume of the backscattering medium (e.g., water) for specified targets and improves upon prior art devices in performance as a result of having more energy in each laser pulse (due to simultaneous operation of multiple lasers) and a more sensitive detection system using multiple cameras. The several cameras may be utilized to image different range gates on a single laser pulse or several cameras can be gated on at the same time to provide independent pictures which can then be averaged to reduce the noise level and improve sensitivity. Both of these improvements result in higher signal-to-noise ratio and thus higher probability of detection or greater range of depth capability.
U.S. Pat. No. 4,963,024 (which is also assigned to the assignee hereof and incorporated herein by reference) relates to a laser based optical system of the type described above used for remote detection of the diffuse attenuation coefficient or K.sub.s factor over very large areas and over relatively short time periods. In accordance with U.S. Pat. No. 4,963,024, a laser is used to generate short pulses (e.g., about less than 10 nm) of light with pulse widths on the order of nanoseconds. The laser light is expanded by optics and projected down toward the surface of the water. An intensified CCD (charge coupled device) camera is electronically shuttered after an appropriate time delay such that the image formed by the camera is composed of light backscattered by the water from a layer of range z and a thickness of delta z. The signal detected by the camera is S.sub.i. If several measurements of S.sub.i are made at known time delay differences such that the differences of range gates z.sub.i are also known, then K can be calculated. This just described optical system thus functions as a K-meter.
Other patents related to imaging lidar systems include U.S. Pat. Nos. 5,029,009 and 5,034,810, both of which are assigned to the assignee hereof.
U.S. Pat. application Ser. No. 632,377 filed in Dec. 21, 1990, and U.S. Pat. application Ser. No. 632,398 also filed Dec. 21, 1990, are also assigned to the assignee hereof and incorporated herein by reference. U.S. Ser. No. 632,377 relates to a LIDAR system which employs a laser tuned to a wavelength selected to optimize performance. U.S. Ser. No. 632,398 relates to an underwater communications system which employs a laser tuned to a wavelength selected to optimize performance.
It is known in the art to use various types of filters and filter receivers in an attempt to limit optical noise in the above-described systems. The most commonly employed filters are the birefringent, the Lyot filter, and the dielectric filter. However, these filters suffer from several distinct disadvantages. For example, such filters typically have a limited (or narrow) acceptance bandwidth. Since the narrower the acceptance bandwidth, the smaller the acceptance angle, then such prior art filters have limited utility. Thus, at acceptance bandwidths much less than 1-2 nm, the narrow acceptance angle becomes restrictive, particularly in imaging lidar applications.
Known atomic resonance filters (such as disclosed in U.S. Pat. No. 4,292,596) also suffer from several disadvantages. The atomic resonance filters have a relatively long response time which is based on the relaxation time "t", of the atomic system on which they are based (t is greater than 30 ns). Thus, the use of atomic resonance filters as an imaging filter is severely limited by the wavelengths associated with the hyperfine line structure of alkali metal atoms by the requirement of a "pump laser" and a low overall detection efficiency (approx. 0.05%). Further, with atomic resonance filters, there is associated a wavelength upshift which requires that more efficient photodetectors or CCD (charge coupled device) cameras be employed.
While well suited for its intended purposes, the above-described optical systems (e.g., imaging lidar and submarine communications systems) are susceptible to optical noise. This problem is particularly troublesome during daylight operation due to the effects of solar noise.